Organic light-emitting diodes (OLEDs) have received increasing attention over the past decade due to their potential use in inexpensive, large area, high brightness, and flexible color displays. Of particular interest is the miniaturization of OLEDs as nanoscale light emitters would enable ultrahigh density displays and novel optoelectronic device architectures. However, as OLEDs approach shorter length scales, fabrication- and materials-dependent micro- and nanoscopic spatial variations in response may lead to significant device-to-device non-uniformities that compromise overall system performance. For example, a common spatially localized defect in OLEDs is the formation of poorly understood “dark spots” at various device locations which exhibit little or no electroluminescence. These dark spots have been reported to increase in size as a function of time, especially in the presence of moisture. To overcome these performance and reliability limitations, a better understanding of common OLED failure mechanisms is required.
Previous attempts to spatially map light emission from organic light-emitting materials include electroluminescence microscopy and near-field scanning optical microscopy (NSOM). In electroluminescence microscopy, light emission from operating OLEDs is detected with an optical microscope. Although this technique is effective at characterizing millimeter scale device features, its spatial resolution is inherently limited by the wavelength of the emitted light. While NSOM achieves sub-100 nm spatial resolution by detecting light in the near-field with a sub wavelength aperture in a fiber tip and has been widely used for characterizing photoluminescence in organic materials, its experimental geometry is not particularly well suited for detecting electroluminescence from operating OLEDs. Importantly, neither technique is capable of simultaneously probing topology, current-voltage response, and electroluminescence-voltage response at the nanoscale.
Atomic force microscopy is described generally in U.S. Pat. No. 6,642,517, the entirety of which—and, in particular, FIGS. 1-2, 4 and 6-7 and corresponding descriptions thereof and the references cited therein—is incorporated herein by reference. More specifically, conductive atomic force microscopy (cAFM) has recently proven to be an effective method for probing current flow and resistivity variations with nanometer scale spatial resolution in gold nanowires, silicon field effect transistors, individual organic molecules, conducting polymer blends, and emissive polymers. See, respectively: M. C. Hersam, A. C. F. Hoole, S. J. O'Shea, and M. E. Welland, Appl. Phys. Lett. 72, 915 (1998); P. De Wolf, W. Vandervorst, H. Smith, and N. Khalil, J. Vac. Sci. Technol. B 18, 540 (2000); A. M. Rawlett, T. J. Hopson, L. A. Nagahara, R. K. Tsui, G. K. Ramachandran, and S. M. Lindsay, Appl. Phys. Lett. 81, 3043 (2002); J. Planès, F. Houzé, P. Chrétien, and O. Schneegans, Appl. Phys. Lett. 79, 2993 (2001); and H. -N. Lin, H. -L. Lin, S. -S. Wang, L. -S. Yu, G. -Y. Perng, S. -A. Chen, and S. -H. Chen, Appl. Phys. Lett. 81, 2572 (2002).
Since the cAFM tip is used to locally inject charge, cAFM can directly stimulate electroluminescence with nanometer scale spatial resolution. With appropriate collection optics and photon detectors, the resulting electroluminescence can be spatially correlated with the cAFM tip position, thus enabling nanometer scale electroluminescence mapping. cAFM is described generally in U.S. Pat. No. 5,874,734, the entirety of which is incorporated hereby by reference. Thus, cAFM and analogous scanning tunneling microscopy measurements have been used to spatially map electroluminescence in a variety of organic materials. However, in these studies, the conductive tip was brought directly into contact with the organic material. While such direct electrical contact with the materials is sufficient to induce electroluminescence, a point contact of this type is inevitably different from the evaporated electrical contacts fabricated in actual OLED devices.